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74 SMT007 MAGAZINE I SEPTEMBER 2024 ing from two factors. One is the high homol- ogous temperature of tin-bismuth solders that keeps the microstructures continuously chang- ing even at temperatures as low as room tem- perature. e other is the high electromigra- tion effective valence of Bi atoms which results in the Bi-rich phase accumulating and segre- gating on the anode side of the solder joint. Not only does this electromigration phenom- enon increase the electrical resistance of the solder joints, but it is also suspected of making the joint prone to cracking in shock and drop tests. Over the past two years, an iNEMI team has been busy studying Sn-Bi electromi- gration to gain a greater understanding of the phenomenon to help design experiments that can better study the effect of electromigra- tion on the susceptibility of the solder joints to shock and vibration. is paper describes some of the fundamentals of metallurgy and electromigration that the iNEMI team learned and discovered over the two-year study. We hope these fundamentals will be useful to those working to make Sn-Bi solders reliable and useful in first- and second-level electronic packaging. Tin-Bismuth Phase Diagram: Why the Bi-rich Phase Volume Fraction Increases With Time When Aged at Room Temperature Binary Sn-Bi alloy has a eutectic phase dia- gram shown in Figure 1 with a eutectic tem- perature of 138.5°C and the eutectic point at 44 atomic % Bi 2 . When cooled from the liquid state to just below the eutectic temperature, two solid phases form: a Sn-rich phase with 13 at %Bi and a Bi-rich phase that is almost pure bismuth. When slowly cooled to room temper- ature under equilibrium conditions, the com- position of the Sn-rich phase follows the a-b solvus line shown in the phase diagram. Under non-equilibrium, air cooling, the Sn-rich phase may follow the a-c line. With time at room tem- perature, two phenomena will occur: 1. e composition of the Sn-rich phase will move from point c to the thermodynamic equilibrium composition point b. 2. e microstructure will coarsen with the larger Bi-rich particles growing at the expense of the smaller Bi-rich particles. e coarsening of the microstructure, also known as Ostwald ripening, is explained in Figure 2 3 . e Bi concentration in the Sn-rich Figure 1: Su-Bi equilibrium phase diagram. Figure 2: Illustration of Ostwald ripening in Sn-Bi alloy. The a-b solid line represents the Bi concen- tration profile in the Sn-rich phase between the Bi-rich particles. The gradient of the Bi composi- tion represented by the a-b line drives the diffusion of Bi atoms from the smaller to the larger Bi-rich particles. The dotted line at 2.9 at.% represents the composition of the Sn-rich phase when, with aging, the Bi-rich particles coarsen to larger sizes.